Residual Magnetic Anomalies Research Articles

Gravity and magnetic study of the Xitle volcanic field, Basin of Mexico – Inferences on the pre-eruptive topography

Results of a gravity and magnetic study over a lava field in the southern Basin of Mexico are used to investigate on lava emplacement and pre-eruptive topography. The characteristics of lava flows depend on multiple factors, including the lava composition, effusion rate, rheology, fluid content and underlying terrain. The ∼2000 years old volcanic field, formed by seven flow units emplaced by the Xitle eruption, covers an extensive area >80 km2 characterized by topographic relief on the Ajusco volcano flank and surrounding plain. The study is conducted on the northern end of the volcanic field. The gravity and magnetic study is based on regional-residual separation, vertical and horizontal derivatives, reduced to the pole, analytical signal, spectral analysis and forward modeling. The regional gravity field shows two anomaly lows in the central sector, with the residual field showing trends of gravity highs. The residual magnetic anomaly is characterized by trends of magnetic highs associated to the lava field. Depths estimated from spectral analysis of gravity and magnetic anomalies show shallow sources at ∼22 m and ∼46 m, and deep sources at ∼469 m and ∼268 m, respectively. The underground sequence is modeled as a volcanoclastic succession of varying thickness. The first vertical derivatives, gradients and analytical signal enhance the anomalies associated to lava thickness variations. Joint gravity and magnetic analysis constraints the deep/shallow anomaly sources in the volcanic field and relations to pre-eruptive structure.

Determination of borders of structure related to deep minerals based on potential field data in Ba Na area

This paper presents the research results of applying method two-dimensional energy filtering method in the «live window» to separate the high-frequency component of aeromagnetic data in order to eliminate residual flight line errors with amplitude ranges from -5÷5 nT and correct the data source before processing and analysis of data, identify potential field residual anomalies at depths h = 470 m, 1100 m, 1700 m, 2300 m. Application of the horizontal gradient of the vertical derivative of the potential field at various depths to determine the position of shallow fault systems, and determine the boundaries of local heterogeneous blocks related to the prospect of deep-hidden minerals at depths h = 470 m, 1100 m, 1700 m, 2300 m in Ba Na area. The results of the analysis of residual magnetic anomalies have built the distribution diagram of the shallow fault system buried by sediments and zoning the mineral prospect according to the potential anomaly in the study area. Fault system includes faults in sub-latitude, sub-longitude, and Northeast-Southwest directions, in which the sub-latitude fault system is dominant. The area of mineral prospect in Ba Na area to a depth h = 2300 m is determined by the maximum value of horizontal gradient vertical derivative of the closed loop residual gravity anomaly field with amplitude >0.007 mGal/m2, combined with residual gravity anomaly value, and location of mineral points have been revealed on the surface, prospective areas are mainly concentrated in the west, northwest, and southeast of the study area. The results of the analysis provided Geophysicists and Geologists with detailed research plans to evaluate specific types of minerals in the Ba Na area.

Application of Euler and Werner deconvolution techniques in delineating tin deposit in Okeso area, Southwestern, Nigeria

The Okeso pegmatites field was evaluated using magnetic method within abandoned mining site close to the southeastern region of Ojoku area. The study aims to delineate, identify geological boundaries and determine depth of tin deposits in the area. The acquired magnetic data was enhanced with the combination of 3D Euler and Werner deconvolution methods. Five magnetic profiles were selected and carried out on the residual anomalies around the abandoned mine site at Okeso with three of the profile lines were in the NW-SE direction and two in the NE-SW direction. The residual magnetic anomalies showed the magnetic susceptibilities of ≤ 4.5118 × 10−3 nT in the ore bearing pegmatite within the basement rocks. Results from profiles of Werner deconvolution identified some shallow tectonic structures like fractures, pegmatites and faults which are capable of hosting metallic tin deposits. The depth to the magnetic source varies from a minimum to a maximum of 300 m to 1200 m below the subsurface in all selected profiles, suggesting the shallow nature of the magnetic source in the area. Additionally, the dip angle ranges from 5.60 to 81.20, potentially attributed to Pan-African shallow structures according to the contact model. Solutions obtained from the structural index of contact and dyke reveals the presence of dyke formation and boundaries which separate rocks from one another. The trend of the lineaments/ fractures which were likely established during the Pan-African orogeny is dominant in the NE-SW direction, conforms with the trends obtained for basement structures in previous studies. Depth range produced by 3D Euler deconvolution is from 50 - 1000 m for all the lineaments. This gives an insight of approximate depth range of all the lineaments/ fractures across the whole map in the study area unlike, Werner deconvolution which is profile biased. The identical signature from all profiles implies that the tin deposit is relatively uniform, extending to a great depth in the area. This represents economically viable quantity and makes it a worthy target for investors.

Application of Euler and Werner deconvolution techniques in delineating tin deposit in Okeso area, Southwestern, Nigeria

The Okeso pegmatites field was evaluated using magnetic method within abandoned mining site close to the southeastern region of Ojoku area. The study aims to delineate, identify geological boundaries and determine depth of tin deposits in the area. The acquired magnetic data was enhanced with the combination of 3D Euler and Werner deconvolution methods. Five magnetic profiles were selected and carried out on the residual anomalies around the abandoned mine site at Okeso with three of the profile lines were in the NW-SE direction and two in the NE-SW direction. The residual magnetic anomalies showed the magnetic susceptibilities of ≤ 4.5118 × 10−3 nT in the ore bearing pegmatite within the basement rocks. Results from profiles of Werner deconvolution identified some shallow tectonic structures like fractures, pegmatites and faults which are capable of hosting metallic tin deposits. The depth to the magnetic source varies from a minimum to a maximum of 300 m to 1200 m below the subsurface in all selected profiles, suggesting the shallow nature of the magnetic source in the area. Additionally, the dip angle ranges from 5.60 to 81.20, potentially attributed to Pan-African shallow structures according to the contact model. Solutions obtained from the structural index of contact and dyke reveals the presence of dyke formation and boundaries which separate rocks from one another. The trend of the lineaments/ fractures which were likely established during the Pan-African orogeny is dominant in the NE-SW direction, conforms with the trends obtained for basement structures in previous studies. Depth range produced by 3D Euler deconvolution is from 50 - 1000 m for all the lineaments. This gives an insight of approximate depth range of all the lineaments/ fractures across the whole map in the study area unlike, Werner deconvolution which is profile biased. The identical signature from all profiles implies that the tin deposit is relatively uniform, extending to a great depth in the area. This represents economically viable quantity and makes it a worthy target for investors.

Application of Airborne Magnetic Data to Assess the Structures that Favour Gold Mineralization within Southern Parts of Katsina State, Nigeria

The Nigerian schist belts are endowed with gold mineralisation and to delineate the geological structures that favour gold mineralisation, the airborne magnetic data that cover the study area was employed. Also, the residual magnetic anomaly data of the research area was reduced to the Equator (RTE) followed by the application of the centre for exploration targeting (CET) plug-in and three source edge mapping techniques; analytic signal (AS), first vertical derivative (FVD) and the horizontal gradient magnitude (HGM), for structural delineation and subsequently the structural density map of the area was produced. Structural density varies from 0.186 km/km2 low to 0.93 km/km2 high. Portions within the study area with high structural densities are Bakori in the southwest, Shanono in the northeast, Gwarzo situated directly south of Shanono, north and south of Karaye, the border between Kafur and Rogo in the south, all within the schist. Since structures are conduits for the migration of mineralisation fluids, areas with high structural densities will be viable for gold mineralisation. Also, comparing the structural density map of the area to the analytic signal map of the area has revealed the high structural density portions to coincide with the high analytic signal zones. This implies that structures within these portions are responsible for the migration of mineralization fluid into these environments.

Magnetic data modelling of salt domes in Eastern Mediterranean, offshore Egypt

We analyzed magnetic data of a deep-water area in the Eastern Mediterranean, offshore Egypt where there are numerous salt domes. Regional-residual separation of the reduced to pole (RTP) magnetic anomalies was performed with the discrete wavelet transform, and the residual magnetic anomalies were interpreted as mainly due to the susceptibility contrast between sediments and salt bodies. A multiscale boundary analysis of the anomalies yielded meaningful lineaments at both large and short scales. Salt bodies lineaments are very well defined in the short-scale map. Then, we inverted the data by a nonlinear and non-iterative 3D inversion technique, to delineate the salt bodies and estimate their magnetization contrast. We used the depths obtained from the Euler deconvolution to form a set of maxima and minima constraints in the inversion. Our procedure yielded an interesting map of the top of salt in the deep-water region, entirely based on the magnetic data, which agrees well with the seismic top of salt. This is a somewhat surprising result, in some respects not obvious, demonstrating that magnetic prospecting can be an important and low-cost tool for the exploration of salt diapirs, especially when seismic data are partially or completely inaccessible or of poor quality.

Magnetic Structural Depth Classification Using Center for Exploration Targeting And 3D Euler Deconvolution: A Case Study of Shanga (Sheet 96) North West, Nigeria

The present study involved the use of 3D Euler deconvolution to classify the depth of occurrences of magnetic structures as explicated and elucidated by the Centre for Exploration Targeting Plug-in (CET). To achieve this aim, the residual magnetic anomaly was first reduced to the equator (RTE) to centre the anomaly peaks directly above each causative source. After that, the RTE data was imputed into the CET plug-in, which is an extension of Oasis Montaj, to delineate structures that occur within the area. A 3D Euler deconvolution map with a structural index of one (SI = 1) that identified faults, dykes, and horizontal bodies was created using the RTE data. The Euler deconvolution depths were classified for depths that are <150 m, between 150 m and 300 m, 300 m and 450 m, and > 450 m. The CET delineated structures were superimposed over the 3D Euler deconvolution map with SI = 1, and most of the CET structures coincided with the structures from the Euler deconvolution map with SI = 1. Since Euler solutions are classified based on depth, the Euler structural map was used to classify the CET delineated structures based on their depths. The Euler structural map was imported into ArcGIS version 10.8 to digitize the structures based on their corresponding depth as displayed on the 3D Euler deconvolution map of SI = 1. After digitizing the Euler map with a structural index equal to one (SI = 1) with classified depth, a new structural map was then produced depicting each structure delineated with the CET based on their respective depth.

Results of high-resolution technologies applied in the acquisition of seafloor information in the Colombian Caribbean Sea

We expound on a hydrographic and geophysical survey in the south of the Archipelago of San Andres, Providencia, and Santa Catalina (ASAPSC) in the Colombian Caribbean Sea. In 2017 and 2018, high-resolution data were acquired from the Caribbean Center for Oceanographic and Hydrographic Research (CIOH). The hydrographic data were processed and filtered per International Hydrographic Organization (IHO) standards and the geomagnetic data were processed, corrected, and filtered to improve the interpretation. We could then characterise the submarine relief, analyse the geomagnetic anomalies in the area, and identify different relief forms of volcanic origin. Regarding geomagnetic work, a spectral analysis of the anomalies revealed highly magnetic bodies in deep water and residual magnetic anomalies strongly related to the seabed morphology. Magnetic highs associated with the basement highs and volcanic cones were detected. Complementary spectral analysis showed that the shallowest magnetic sources were in the first 500 m, whereas the deepest magnetic sources were between the depths of 4 and 5 km. The geomagnetic data were also validated using Euler deconvolution analysis, which confirmed these magnetic anomalies. Thus, a direct relationship between the magnetic anomalies and bathymetry of the seafloor was confirmed, reinforcing the theory of the volcanic origin of these islands.

3‐D geophysical modeling of a buried, simple impact crater: Holleford impact structure, Ontario, Canada

AbstractHolleford Crater is a deeply buried, 2.35 km diameter late Proterozoic‐early Cambrian (550 ± 50 Ma) simple impact crater located in southeastern Ontario, Canada. Exploration drilling in the 1950–60s indicated a >450 m deep, simple impact structure with an infill stratigraphy of Cambro‐Ordovician clastic and carbonate sediments and a −2.2 mGal gravity anomaly. We conducted new ground‐based geophysical surveys (magnetics, gravity) and potential field modeling to better resolve the buried impact structure depth, subsurface geometry, and postimpact modification. Geophysical surveys reveal a well‐defined ~3 mGal Bouguer gravity low and <20 nT residual magnetic anomaly over the crater basin. The lack of a well‐defined magnetic anomaly is due to the low magnetic contrast between Mesoproterozoic metasedimentary target rocks and Paleozoic infill sediments. The modeled basement surface shows a deeply buried (>400 m), eroded simple impact structure with a rim‐to‐rim diameter (D) of 1.8–2 km, a residual rim height of about 30 m and a true depth (dt) >400 m. The modeled diameter is smaller than the previous estimate (2.35 km) based on the surface outcrop pattern of Paleozoic strata, which overestimates the buried impact structure dimensions. The modeled basement surface identifies a rim breach and a possible fluvial outflow channel in the southeast impact structure rim. The channel is up to 150 m deep and 400 m in width and has morphology similar to outlet channels produced by fluvial rim dissection of terrestrial impact structures and “tadpole craters” on Mars.

Crustal architecture of the Dharwar craton and Southern Granulite Terrane, Southern India, from the analysis of gravity-magnetic data

The potential field (gravity & magnetic) data, along with results of available seismic and magnetotelluric studies over the Southern India Shield comprising the Dharwar craton (DC), Southern Granulite Terrane (SGT) and southern part of the Eastern Granulite Mobile Belt (EGMB), were synthesized. The study attempts to elucidate the geometry and continuity of greenstone belts/shear zones, isostatic condition and deeper crustal architecture. We observed a characteristic elevation-gravity relationship for each of the tectonic domains. The greenstone belts of the DC are characterized by N-S to NW-SE trending positive residual gravity anomalies and continue beneath the Cuddapah sediments. The basement of the Cuddapah basin is characterized by NW-SE trending greenstone belts with low-density intrusive rocks. The gravity anomaly suggests occurrence of a high-density rock unit occurs at mid-crustal depth beneath the ‘Closepet granite’ and ‘Chitradurga schist belt’. The potential field signatures of the DC and SGT are distinctly different. The Cauvery Shear Zone is characterized by positive residual gravity and magnetic anomalies with ∼E-W to ENE-WSW trend that continues further east under sediment covers of the east coast. The residual gravity signatures and horizontal gravity gradient signatures suggest that the orogenic systems encompassing the Cauvery Shear Zone and the eastern part of the EGMB may be a part of a common structural belt or tectonic regime associated with the assembly of the Gondwana supercontinent. The potential field modelling reveals the presence of a thick, high-density material resting over the base of the crust in the entire stretch of the east-coast of India.

EVALUATION OF UPWARD CONTINUATION AND REDUCTION TOEVALUATION OF UPWARD CONTINUATION AND REDUCTION TO MAGNETIC EQUATOR ON AIRBORNE MAGNETIC DATA MAGNETIC EQUATOR ON AIRBORNE MAGNETIC DATA

This research work is aimed at interpreting the airborne magnetic data of the study area for mapping the basement geologic structures using magnetic image enhancement filters. It focus on performing filtering techniques to expose and identify the magnetic properties of the basement structures at various continuation distances. The procedure applied to Total Magnetic Intensity (TMI) data of the study area using Oasis MontanjTM software were Reduction to Magnetic Equator (RTME) to remove the inclination effect as well as positioning the peak of the anomalies over their sources so as to improve the orientation of the magnetic sources, separation of regional and residual magnetic anomalies. These was followed by application of upward continuation at 500 m, 1000 m, 2000 m and 3000 m height. The results of the RTME shows the amplitude of the magnetic anomalies ranging from -37.577 nT to 27.398 nT. High magnetic anomaly is dominated in the northwest, southeast through southern part of the study location. Furthermore the results of the Upward Continuation (UC) revealed a wider wavenumber anomalies as well as height above the near surface. Hence the results of this research has displayed an enhanced geological magnetic structures located in the study area and also shows an improved quality of magnetic anomalies as the continuation distances increases.

An aeromagnetic denoising-decomposition-3D inversion approach for mineral exploration

Reduction of aeromagnetic noise and extraction of mineralization-related residual anomalies are critical for aeromagnetic data processing in mineral exploration. This study introduced a multifractal singular value decomposition (MSVD) method to remove the noise and improved the bi-dimensional empirical mode decomposition (BEMD) algorithm to extract residual magnetic anomalies. It is shown that MSVD and improved BEMD could effectively reduce the noise and extract residual magnetic anomalies. Then, a wavenumber–domain iterative approach is applied in 3D imaging of magnetic anomalies and gradients with depth constraints, which is a rapid tool for qualitative and quantitative interpretation of magnetic data and is suitable for rapidly imaging large-scale data. The 3D inversion result is verified by four geological sections along the regional tectonic directions and some drilling holes on the deposits. It is revealed that this proposed approach is practical and effective in dealing with aeromagnetic data interpretation and inversion for mineral exploration.

Distribution of magmatic rocks in eastern Yin'e Basin revealed by 3D gravity and magnetic inversion

Magmatic activity has important influence on sedimentary cap structure, hydrocarbon generation and reservoir formation. There are multiple periods of volcanic activities in Yin'e Basin and it is of great significance to study the distribution of magmatic rocks in the basin for the prospective investigation of oil and gas resources. The residual gravity and magnetic anomalies are extracted from the original gravity and magnetic data in the eastern area of Yin'e Basin and used for 3D inversion. The spatial distribution of magmatic rocks in the study area is explained based on the inverted 3D density and magnetic susceptibility. The physical properties of the magmatic rocks in Yin'e Basin are distinctly different from their surrounding rocks in physical properties. The acidic igneous rocks are mainly low-density and medium-high magnetic (a few are weakly magnetic), and the intermediate-basic igneous rocks are mainly high-density and high-magnetic. The local low gravity anomaly with high magnetic anomaly area may be caused by the acidic igneous rocks intruding into the Carboniferous and Permian basement, while the local high gravity anomaly with high magnetic anomaly area may be caused by the intrusion of intermediate-basic igneous rocks. 14 weakly magnetic acidic igneous rocks, 16 medium-strong magnetic acidic igneous rocks and 25 intermediate-basic igneous rocks are inferred from the 3D inverted density and magnetic susceptibility. Acidic igneous rocks, which are distributed in the whole region, have large scale and mainly banded along the NEE direction. The intermediate-basic igneous rocks, which are generally small and most of them are equiaxed, are mainly distributed along the boundary of Zongnaishan-Chulu Uplift and Shangdan Depression, as well as the southern basin periphery of Shangdan Depression. The well-developed acidic magmatic rocks in Suhongtu and Shangdan Depressions, whose scale is relatively large, may be of great significance to the formation and distribution of oil and gas in Yin'e Basin.

CHIBOK RIFTO-MAGNETICS AND GEOLOGY

On sheet 134 of the aeromagnetic map of Nigeria is a prominent northeast (NE) trending negative anomaly that extends for >20 km. This work reinterprets the anomaly using oasis montage™ 7.0 software, by inverse geological modelling of magnetic profiles. The aim of this work is to identify the Chibok magnetic anomaly as a plate tectonic feature, and as part of the West African Rift System. Three profiles were modelled across the residual magnetic anomaly, which yielded a depth range of 1 to 2 km. The result furnished a subsurface picture of ‘horst and graben’ which are characteristic of rifts. The sedimentary infill of the interpreted rift is considered to be sands/or sandstone due to the low magnetic anomaly. Sandstone has very low magnetic susceptibility [30 x106 emu (S.I.)] compared to granite and basalt. Geological data showed NW, NE, N-S, and E-W structural trends on the granitic basement. These manifest as faults, shear zones, joints and foliations. Emplacement of the Tertiary basalt in the NW of the area of study area showed planes of crustal weakness in the Precambrian basement. A palinspatic map of the area identified the pre-Tertiary geology. The sediment infill of the interpreted rift may serve as very rich aquifer for groundwater abstraction for irrigation in this semi-arid zone. The evidence of the plate tectonic nature of the interpreted rift indicated its alignment with offshore transform faults like Chain and Charcot as well as the Cameroun Volcanic Line.

Aeromagnetic Survey as Reconnaissance Technique for Groundwater Exploration in a Typical Southwestern Nigeria Basement Complex

This study was aimed at investigating the causes of reported boreholes failureparts in some part of the Federal Polytechnic Ado Campus using aeromagnetic geophysical method. The aeromagnetic geophysical data covering the area was processed to enhance shallow anomalies using different enhancement techniques such as derivatives and wavelength filters. Total Magnetic Intensity ranges from -28 to 201 nT after removing regional component. Residual magnetic anomaly ranges from -21 to 127 nT; First vertical derivative ranges from -0.18 to 0.36 nT/m; Magnetic Analytic signal (AS) ranges from 0.03 to 0.36 nT/m. It was observed, by correlating the magnetic responses and geology of the area, that rocks underlain the area have close magnetic susceptibility. Consequently, the variations of earth’s magnetism are mainly controlled by sediment thickness and suspected linear structures which are major factors in basement complex groundwater exploration. The probable depth to basement in the study area ranges from 8.13 to 74.05 m as revealed by AS and spectral method, which implies thicker regolith in southern part of the study area than northern part. However, linear structures are evenly distributed across the study area. Therefore, the groundwater potential of the southern part of the study area is higher due to regolith thickness. The failed groundwater boreholes that are prominent in northern part of the study area are not located on linear structures. Therefore, basement depth and linear structures should be considered during reconnaissance survey in groundwater exploration in a typical Basement Complex terrain.

Joint interpretation of gravity and airborne magnetic data along the Calama-Olacapato-Toro fault system (Central Puna, NW Argentina): Structural and geothermal significance

Here, we present new gravity and airborne magnetic geophysical data from a preliminary exploration along the western sector of the NW-striking Calama-Olacapato-Toro fault system. This area is located in the back-arc region of the Central Andes (NW, Argentina) and is considered as a high potential geothermal area within the Central Puna Energy Hub.The gravity and magnetic modelling was carried out with the aim of delineating geological structures that control the geothermal manifestations and estimate the geothermal field extent. Gravity data were separated into regional and residual components. Then, a 2-D gravity and magnetic modelling was carried out through the 2-D GM-SYS - Oasis Montaj software in order to build an inversion model that adjusts the residual gravimetric and magnetic anomalies observed on the surface. The resultant model was also fitted taking into consideration previous electrical resistivity studies, surface manifestations, geology, and geological structures. Bouguer anomalies are negative in the range of -411 to -304 mGal, while magnetic anomalies have a range of − 127 to 15 nT, in the central part of the study area. Hot springs correlate with low residual Bouguer and magnetic anomalies located on the Calama-Olacapato-Toro lineament, and are related to the alteration zone and the main damage zone. Gravity and magnetic inversion show low density and susceptibility contrast of rocks associated with volcano-sedimentary rocks filling a volcano-tectonic depression, limited by the Antuco and Pompeya N-S thrust faults, westward and eastward respectively. The low density and susceptibility contrast can be related to a combination of high temperatures and steam versus liquid-filled pores and fractures. Hence, this geological conceptual model based on the 2-D forward modelling of geophysical data, allows better knowledge and comprehension of hydrothermal fluids circulation in the middle of the Central Puna, in northwest Argentina.

Magnetic anomaly patterns and volcano-tectonic features associated with geothermal prospect areas in the Ziway-Shala Lakes Basin, Central Main Ethiopian Rift

The Ziway-Shala Lakes Basin is a structural depression within the Main Ethiopian rift that is associated with Cenozoic volcanism, faulting and sedimentation. The Aluto-Langano volcano complex contains a geothermal field within this basin that is currently being exploited for geothermal power production. Recent magnetotelluric analyzes did not reveal a heat source beneath the Aluto-Langano geothermal field but found an electrical conductor that may be the magmatic heat source near the Silti Debre Zeyet Fault Zone (SDFZ) northwest of the Aluto- Langano geothermal field. To investigate the subsurface structure of the Ziway-Shala Lakes Basin and especially the Aluto-Langano geothermal field, a ground based magnetic survey was performed. Volcano-tectonic features were identified using a differentially reduced to the pole total magnetic field anomaly and a variety of map transformations including upward continuation, first vertical derivatives and tilt derivatives, to map the regional and residual magnetic fields. To identify subsurface magnetic susceptibility sources between the SDFZ and the Aluto-Shala geothermal field, a power spectral analysis, 2D-Werner Deconvolution, and 2D forward modeling were applied. The integrated interpretation of the maps and models, indicates that magnetic maxima occurred over the SDFZ and the Wonji Fault Zone and the minima over the Aluto-Langano geothermal field. The magnetic maxima may be caused by shallow (< 5 km) igneous intrusions that are related to deeper magma movement. The magnetic minima may be caused by warm geothermal waters causing thermal alteration of the magnetic minerals under the Aluto-Langano volcanic complex. The residual and derivative magnetic anomaly maps suggest the existence of lineations that may transfer geothermal fluids suggested by the magnetotelluric models from the SDFZ to the Aluto-Langano geothermal field. The upward movement of the fluids is evident by the magnetic minimum in the Aluto-Langano geothermal field and agrees with a recent earthquake analysis. Additionally, a majority of the magnetic lineaments are oriented parallel to pre-existing Mesozoic structures related to the opening of the Ogaden Basin.

Aeromagnetic investigation of crustal stability of Osi, north central Nigeri

High-resolution aeromagnetic data of Osi north central Nigeria has been processed with a view to establish the earthquake-prone fault zones in the area. The analytical signal amplitude, shaded relief total horizontal derivative, first vertical derivative and 3D Euler deconvolution maps produced from the residual magnetic intensity anomalies of the study area identified the structural features in the area. The locations and the orientations of the extracted magnetic lineaments were revealed by the structural map of the area and the associated rose diagram. The lineaments were observed to strike NE-SW, ENE-WSW, E-W, NW-SE, WNW-ESE, NNE-SSW and NNW-SSE. The upward continuation maps showed evidence of deep-lying faults while the source parameter imaging and spectral analysis revealed depth to magnetic sources ranging from 81 m to 402 m and 103 m to 460 m respectively. The predominant NE-SW and NNE-SSW trending faults in the study area depicted stress history related to Ifewara faults; which is an indication of possible impending earthquake occurrence in the area.

Geophysical investigation of crustal structural framework of Ilesha, southwestern Nigeria

The interpretation of the aeromagnetic dataset of Ilesha southwestern Nigeria has been carried out to reveal possible future earthquake hazard as well as establish the groundwater potential of the area. The edge detection methods involving the analytical signal amplitude, the total horizontal derivative, the first vertical derivative and the 3D Euler deconvolution were performed on the residual magnetic intensity anomalies to identify the linear structural discontinuities associated with lithological boundaries/ faults in the area. The upward continuation filter and depth estimation procedures of source parameter imaging and radially averaged power spectrum were applied to the residual magnetic intensity data to map geological structures such as faults as well as determine the depth to crystalline basement rocks in the area. The obtained lineaments observed to trend predominantly towards the NE-SW direction indicative of Pan African Orogeny, suggest possible high yield groundwater aquifer zones. The upward continuation filtered maps showed evidence of deep-lying faults to depth beyond 2 km. The 3D Euler deconvolution plots and spectral analysis revealed depth to magnetic sources ranging from 55 m to 345 m and 13 m to 250 m, respectively. The delineated structural discontinuities in the study area revealed stress history similar to Ifewara-Zungeru fault network, suggestive of possible future earthquake occurrence in the area. It is however recommended that further geophysical studies should involve electrical method to establish the groundwater potential as well as the stability of the basement underlying the area.

Support vector machine-based integration of AVIRIS NG hyperspectral and ground geophysical data for identifying potential zones for chromite exploration – A study in Tamil Nadu, India

In the present study, we have integrated the spectral anomaly map derived from airborne visible-infrared imaging spectrometer-next generation (AVIRIS-NG) data and residual ground geophysical anomaly maps (gravity and magnetic) for identifying the mafic cumulates and chromitites in the Sittampundi Layered-Complex (SLC), Tamil Nadu, India. A spectral map of chromite bearing mafic cumulates was prepared by implementing the matched filtering (MF) algorithm on the selected spectral bands of AVIRIS-NG data using the mixed image spectra of mafic cumulate and chromitite as an end member. A low pass filter was applied in the spectral map to extract the clusters of coherently distributed spectrally anomalous pixels representing the surface exposure of mafic cumulates. Concurrently, the ground gravity and magnetic data were processed for deriving the residual gravity and magnetic anomaly maps. Subsequently, an integration of spectral map, residual gravity, and magnetic anomaly maps was attempted using the support vector machine (SVM) algorithm to identify the potential sites of chromitite occurrence. Results were verified using the geological map and the field data on surface exposures of mafic cumulates. This research demonstrates that integration of remote sensing data and geophysical anomaly maps has immense potential to detect the surface distribution of chromite bearing mafic cumulates in the layered complex and the same methods can be replicated for carrying out mineral prospecting in the other parts of the world.